Remove incorrect __restricted labels from strcspn

strcspn should never have had the __restrict keywords. While both of
these strings are const, it may have unindended side effects. While this
is the kernel, the POSIX definition also omits restrict.

Reviewed by: imp@
Pull Request: https://github.com/freebsd/freebsd-src/pull/497

(cherry picked from commit 56d33e86b74b197a36f42255824b56715c96a596)